The MEMS microphone, also called MEMS transducer, refers to a microphone fabricated with Microelectromechanical System technology.
The microphone contains two chips, a MEMS die and an Application Specific Integrated Circuit (ASIC) chip which are packaged into a surface mountable package. The MEMS die contains a rigid perforated back electrode and an elastic silicon diaphragm serving as a capacitor which can transform the sound wave into the capacitance changes. The ASIC chip is used to detect the capacitance changes and output electrical signals.
Compared with the conventional Electret Condenser Microphone (ECM), the MEMS microphone has not only a very good acoustic performance but also a higher SNR and a more consistent sensitivity, as well as a very stable performance while subjected to various temperatures. Another prominent advantage of the MEMS microphone is that its power consumption is very low, only 70 μW in average with an operating voltage range 1.5V˜3.3V. Moreover, compared with the conventional ECM, it is easier for the MEMS microphone to be incorporated into a microphone array which has a high stability. Combined with the consequent voice algorithm, the microphone array can achieve the voice directivity and improve the communicating quality. Based on above features, the MEMS microphones are widely used in smart phones, consumer electronics, laptops, medical apparatuses such as hearing aids, vehicle industry such as hands-free communicating devices, even in other industry fields as well, such as monitoring machine running state with a sound wave transducer.
The microphone directivity refers to the receiving capacity of the microphone to sounds from different angles. More particularly, the directional capacity is normally reflected by the directional angle. If the directional angle of a microphone is larger, the pickup range is wider but it also has a higher risk of howling caused by picking sounds from loudspeaker field. However, if the directional angle is decreased, the pickup range of the microphone is also decreased correspondingly.
Referring to FIG. 1a and FIG. 1b, the detailed structure of the existing directional MEMS microphone is depicted, comprising: PCB 1, acoustic port 2, MEMS die bonding adhesive 3, metal wire 4, ASIC chip 5, ASIC bonding adhesive 6, ASIC coating adhesive 7, microphone cover 8, cover bonding adhesive or solder paste 9, MEMS die 11, damping adhesive 12, damping 13, acoustic port 10, and pad 20. The PCB 1 is secured on the mounting position by the pads 20. The microphone cover 8 is coupled to the PCB 1 to form a housing with the joint sealed by the cover bonding adhesive or solder paste 9. Inside the housing, the ASIC chip 5 is attached to the PCB 1 by the ASIC bonding adhesive 6 and is packaged by the ASIC coating adhesive 7. The MEMS die 11 is attached to the PCB 1 by the MEMS die bonding adhesive 3. The PCB 1 is provided with the acoustic port 2 and the acoustic port 10 wherein the inner outlet of the port 10 is covered by the damping 13 which is attached along the edge to the PCB 1 by the damping adhesive 12. The outside sound pressure or air pressure through the acoustic port 2 and the acoustic port 10 induces vibrations in the diaphragm over the MEMS die 11. The ASIC chip 5, MEMS die 11 and the PCB 1 are electrically interconnected by the metal wires 4 to input the signal to the PCB1 and then output the signal. The inventor of the present invention found that the existing technology has the following technological issues at least during achieving the present invention.
The sensitivity is a significant performance index of the directional MEMS microphone. For the above existing MEMS microphone, the sensitivity depends on the sound pressure entering the inside of the MEMS microphone to act on the diaphragm 11 through the acoustic port 2 and the acoustic port 10, respectively. As a result of size restriction, the back volume formed on the bottom of the MEMS microphone and the back cavity formed between the cover and the PCB are small, so the channels of the two acoustic ports can't be designed long enough, which leads to the sound pressure difference acting on the diaphragm 11 is minor, thereby the sensitivity difference being minor as well. Thus, sounds from various directions can all enter the inside of the MEMS microphone, and the background noise can't be eliminated by the means of sensitivity difference.